1. Field of the Invention
The present invention relates to a piezoelectric device, and more particularly, to a piezoelectric device which has a plurality of piezoelectric resonators.
2, Description of Related Art
Well-known piezoelectric devices include a type which has a piezoelectric substrate supporting a plurality of piezoelectric resonators. In order to reduce the size of this type of piezoelectric device, a piezoelectric substrate of a smaller size must be used, and accordingly, the intervals between the piezoelectric resonators become smaller.
When the piezoelectric resonators are disposed at short intervals, interference is likely to occur among the resonators, thereby lowering the spurious characteristic of the piezoelectric device. When a voltage is supplied to the piezoelectric resonators, each of the piezoelectric resonators generates a primary vibration in a direction of the thickness of the piezoelectric substrate and also a secondary vibration in a direction along the surface of the piezoelectric device. The secondary vibration is propagated on the surface of the piezoelectric device in the form of an oscillatory wave and is attenuated gradually. However, if the intervals between the piezoelectric resonators are short, the secondary vibration generated by one resonator reaches an adjacent resonator before being attenuated sufficiently and interferes with the primary vibration generated by the adjacent resonator. Thus, the piezoelectric device does not have a good spurious characteristic
In the light of the problem, Japanese Utility Model Laid Open Publication No. 58-11316 disclosed a piezoelectric device wherein two piezoelectric resonators are arranged on a piezoelectric substrate such that secondary vibrations generated by the respective resonators are propagated in perpendicular directions which are at angles of about 45 degrees to the peripheral edges of the piezoelectric substrate. This structure is to prevent direct interference between the secondary vibrations of two piezoelectric resonators by arranging the resonators out of the propagation paths of the secondary vibrations generated by each other. In the structure, however, the propagation paths of the secondary vibrations of the two piezoelectric resonators intersect, and the secondary vibrations interfere with each other at the intersection. Then, the interference causes an undesired oscillatory wave, and the undesired oscillatory wave interferes with the primary vibrations of the piezoelectric resonators.
Because of the interference of the undesired wave with primary vibrations, the structure to prevent the direct interference between secondary vibrations is not so effective to improve the spurious characteristic as might have been expected.